Context: I've put thousands of hours into various other ARPGs - all the Diablo games, Torchlight, Grim Dawn etc. 40K Inquisitor Martyr is a really good game, but there are some fairly minor changes that would be big from a quality of life (QoL) perspective.
Allow anyone who has completed the campaign on one character to both skip the tutorial missions on all subsequent characters (even if a different class) and start the game with all vendors unlocked. Once you've done them once, the tutorial and campaign are extremely boring and unrewarding compared with other activities.
Allow the option to show Relic rarity item drops on the minimap, so that people speedrunning or playing co-op and split farming levels don't miss good gear. Diablo 3 does this and it's very handy.
Once you hit level 50, allow respeccing of passive skill trees without using the Mind Reset consumable (just right click a point to remove it). Mind Reset is trivially cheap at that point in the game but having to get one out, use it, wipe the skills from a single tree and then re-do the whole tree is tedious and pointless.
Make Psyker buffs that reserve warp heat but have a cooldown much shorter than their duration (e.g. Warp Speed) toggle-able so they don't have to be reapplied every 90 seconds or whatever. This wouldn't apply to something like Death Augury where the cooldown prevents you from having it active after it's been popped, obviously.
Add a training dummy to the game so people can test damage with different setups and combos. This may not be as easy to do as the other suggestions.
Add a warning to the 'Abandon mission' button during Void Crusades so that it's clear that abandoning a single mission will abandon the entire Crusade. It currently reads like you're only abandoning the mission (costing you some lives) but you lose the whole run.